The long-term goal of this research is to understand the nature and mechanism of action of factors that cause and modify the course of autism and related disorders. It has recently been learned that autistic deficits exist along a continuum of severity in the general population, comprising what has been referred to as the "broader autism phenotype." Measurement tools capable of reliably ascertaining autistic symptoms as quantitative traits have only recently become available. One such tool, the Social Responsiveness Scale (SRS--formerly known as the Social Reciprocity Scale) is a 65-item inventory (requires 15-20 minutes to complete), which ascertains scores for symptoms in each of the 3 criterion domains of the autistic phenotype. Scores on the instrument distinguish children with autism spectrum disorders from those with other child psychiatric disorders. In previous studies, reciprocal social behavior, as measured b the SRS, has been found to be highly heritable, and genetic influences on social deficits measured by the SRS have been found to be largely independent of genetic influence on other domains of psychopathology in children. In this prospective study, the SRS will be used to examine stability and change in autistic traits over 5-year period, in clinical (n=275) and epidemiologic twin (n=1350) samples. The life course of autistic traits will be examined across the entire range of severity in which such traits occur in nature. The effects of genetic and environmental influences on change in severity over time will be explored, and prognosis in affected children will be studied as a function of severity, age, gender, I.Q., SES, comorbid psychiatric conditions, and treatment conditions. The factor structure of sub threshold autistic traits, and the extent to which they aggregate in the family members of affected individuals, will also be explored.